Thermodynamic Aspects of Polymer Mixtures

There have been a great many studies on polymer mixtures both from a scientific and an engineering viewpoint and they have been reviewed by many authors1–3 and in many books.4–6 However, it is only recently that polymer mixtures have been shown to be analyzable by thermodynamics. One of the many reasons for this situation is that polymer mixtures had previously been believed to be incompatible with each other due to the small entropy of mixing for long polymer chains and positive energy of mixing usually observed between polymers. However, due to rapid progress in experimental techniques used to study the state of polymer mixtures such as small angle neutron scattering (SANS),7 small angle X-ray scattering (SAXS),8 pulsed nuclear magnetic resonance (NMR),9,10 Fourier transform infrared absorption (FT-IR),11 thermal analysis.12 excimer fluorescence,13 etc., quite a few polymer mixtures showing compatibility in molecular or segmental scale have been found. Moreover, some polymer mixtures have been shown to exhibit lower critical solution temperature (LCST)-or upper critical solution temperature (UCST)-type phase diagrams for liquid/liquid equilibrium and melting point depression for liquid/crystalline polymer mixtures. Therefore, it becomes quite natural to treat these problems thermodynamically as extensions of polymer solution theories or theories for metallically.2,14,15